Run unit tests in live browsers

@ascott1 Because all our OAH modules are used client-side, we should really be running our unit tests in real-world browsers to properly spot bugs (I'm looking at you IE8).

Using Karma as a runner with the Saucelabs API, it's possible to have Travis do it automagically without having to change any of our nodeunit tests. We just have to add a couple config files. Try it out locally and we can discuss this strategy tomorrow. Don't merge this PR.

1. Check your email for the sauce credentials.
1. Pull down and merge the below `overall-loan-cost` changes locally.
1. `npm install -g karma-cli browserify`
1. `npm install`
1. `npm run browser-test`